1. Field of the Invention
The present invention relates to a method for producing steel suitable for grain-oriented electrical steel sheet or strip by an oxygen-blown convertor (hereinafter called simply "converter").
2. Description of the Prior Art
A grain-oriented electrical steel sheet having excellent magnetization characteristics in the rolling direction can be obtained by controlling the so-called secondary crystallization phenomenon in which the primary recrystallization grain having {110}&lt;100&gt; orientation grows selectively, and thus, by increasing the proportion of the grains having {110}&lt;100&gt; orientation during the final annealing. For this purpose, it is necessary particularly to form a precipitated dispersion suitable for the secondary recrystallization. As for the impurity elements for forming this precipitated dispersion, one or more of Mn, S, Al, Se, N, Sb, V, Cu, P, Ti and Cr are used and, among them, Mn is the most commonly used.
In order to effect full solid dissolution of the MnS as is commonly done during the slab treating step, it is necessary to maintain the content of Mn to less than a certain value. On the other hand, in some cases, MnS is not used positively for forming the precipitated dispersion, and in such cases, it is necessary to further lower the content of Mn. It has been found from the above facts that a manganese content of not more than 0.075% in the steel is desirable. In actual practice, however, since this value is a ladle analysis and pick-up of Mn is caused in the ladle, the content of Mn as a blown-off Mn content should be less the above value.
In recent years the convertor steel-making process has been largely and actively introduced for refining molten steel, and various advantages are thought to be obtained by oxygen-blowing steels for grain-oriented electrical steel sheets in a convertor.
For molten steel having a low blown-off Mn content as mentioned above by the conventional convertor blowing technics, it is natural that the [O] content in the steel is high, so that inclusions increase in the steel.
If a large amount of inclusions is present in the molten steel, a poor precipitated dispersion of MnS is obtained and unsatisfactory secondary recrystallization takes place, so that the magnetization characteristics deteriorate. Thus it is necessary to avoid increase of the [O] content in the molten steel prior to deoxidation, and for this purpose, it is necessary to maintain a slightly high blown-off content of C depending on the C-O equilibrium.
Thus under the above situations, studies have been made of the convertor steel-making process in order to solve the above problems for preparation of molten steel for grain-oriented electrical steel sheet, and as a result, attempts have been increasingly made to produce such steel in a convertor.
For example, a Japanese patent publication discloses a convertor blowing process, wherein intermediate slag-off is performed while maintaining the bath temperature in the range of 1350.degree. and 1450.degree. C. at a carbon content between 1.5 and 3.0% in the molten steel in the first blowing stage. Then, in the second blowing stage, a soft blowing is performed at a point wherein the carbon content in the molten steel is not less than 0.05% to obtain a final Mn content of not more than 0.055%. By this method, the magnetic property levels required were satisfied and advantages deriving from the convertor processes were effectively utilized. However, the following disadvantages accompanied the process.
According to the above prior art, a low-Mn steel is obtained by increasing the MnO % in the slag in the second blowing stage, and the soft blowing is performed for the purpose of increasing the percent of FeO in the slag, so that the [O] content in the molten steel is increased. Thus, it is not possible to completely and stably obtain a final product having a magnetic property level as required for present day uses. Further, it requires a skilled technic to control the Mn content to a predetermined low value by the soft blowing in the convertor, and the Mn content often deviates from the predetermined value and thus reblowing is compulsory. However, reblowing causes an increase in the N content in the steel and the following serious problems are caused.
In recent years, "a continuous casting process" in which molten steel is continuously cast into molds has been adopted for the purpose of improving productivity and assuring uniform steel quality. But, if the content of nitrogen in the molten steel to be continuously cast is high, local swellings called blisters are caused after the cold rolling and heat treatment in the production process of a grain-oriented electrical steel sheet and the blisters damage the appearance of the final products and lower the space factor of the steel sheet.
Still further, as the soft blowing is performed in order to obtain a low-Mn steel and the intermediate slag-off is performed prior to the soft blowing (the second blowing stage), the tapping yield is lowered and the blowing time is increased.